1. Field of the Invention
This invention relates to a process for manufacturing a micro-structured optical fibre and to a method for producing a structured intermediate preform to be used in such a process.
2. Description of the Related Art
Optical fibres are used for transmitting light from one place to another. Normally, optical fibres are made of more than one material. A first material is used to form a central light-carrying part of the fibre known as the core, while a second material surrounds the first material and forms a part of the fibre known as the cladding. Light can become trapped within the core by total internal reflection at the core/cladding interface.
These conventional fibres are typically produced by well-known vapour deposition techniques, such as MCVD (Modified Chemical Vapor Deposition), OVD (Outside Vapor Deposition) and VAD (Vapor-phase Axial Deposition).
A more recent type of optical fibre waveguide, having a significantly different structure from that of conventional optical fibres, is the micro-structured fibre (also known as “photonic crystal fibre” or “holey fiber”). A micro-structured optical fibre is a fibre made of a same homogeneous material (typically silica), having inside a micro-structure (i.e. a structure on the scale of the optical wavelength) defined by micro-structural elements extending longitudinally along the fibre and having a predetermined distribution. As a micro-structural element it is possible to identify a micro-hole or a filiform element of a different material than the bulk.
The most common type of micro-structured optical fibre has a cladding region showing a plurality of equally-spaced tiny holes, surrounding a homogeneous and uniform central (core) region. A fibre of this type is described, for example, in international patent application WO 99/00685. In a different embodiment, the central region of the fibre may have a central hole, as described, for example, in international patent application WO 00/60388.
These two types of fibres convey light in the core according to different optical phenomena.
In the absence of a central hole, propagation of light in the cladding region is forbidden due to the presence of a lowering of the average refractive index with respect to the core region. Such a structure forms a low-loss all-silica optical waveguide that, for appropriare parameters, remains monomode for all wavelengths within the transmission window of the silica. The waveguiding mechanism in that case is closely related to that in conventional optical fibres and is a form of total internal reflection between two materials (air and silica) having different refractive indexes.
In the presence of a central hole, propagation in the cladding region is forbidden due the presence of a “photonic band-gap”. The “photonic band-gap” phenomenon, which is analogous to the “electronic band-gap” known in solid-state physics, avoids light of certain frequencies to propagate in the zone occupied by the array of holes, this light being therefore confined in the core region. Propagation of light in fibres showing a photonic band gap is described, for example, in J. C. Knight, J. Broeng, T. A. Birks and P. St. J. Russell, “Photonic Band Gap Guidance in Optical Fibres”, Science 282 1476 (1998)).
Optical characteristics of the above-described micro-structured fibres depend on the number of holes, the holes diameter, the reciprocal distance between adjacent holes and the hole geometrical pattern. Since each of these parameters can broadly vary, fibres of very different characteristics can be designed.
Micro-structured optical fibres are typically manufactured by the so-called “stack-and-draw” method, wherein an array of silica rods and/or tubes are stacked in a close-packed arrangement to form a prefom, that can be drawn into fibre using a conventional tower setup.
In U.S. Pat. No. 5,802,236A, for example, a core element (e.g., a silica rod) and a multiplicity of capillary tubes (e.g., silica tubes) are provided, and the capillary tubes are arranged as a bundle, with the core element typically in the center of the bundle. The bundle is held together by one or more overclad tubes that are collapsed onto the bundle. The fibre is then drawn from the thus prepared preform.
A different stack-and-draw method is described in the above-cited patent application WO 99/00685, and comprises:                producing a cylindrical rod of fused silica;        drilling a hole centrally along the length of the rod;        milling the outside of the rod to obtain six flats so as to confer to the rod a hexagonal cross section;        drawing the rod into a cane by using a fibre drawing tower;        cutting the cane into the required length;        stacking a plurality of such canes to form a hexagonal array of canes, the cane at the centre (that will define the core of the fibre) having no hole drilled through the center; and        drawing the stack of canes into the final fibre using the fibre drawing tower.        
The Applicant has noted that the stack-and-draw manufacturing method has several drawbacks.
The awkwardness of assembling hundreds of very thin canes (defined by rods or tubes), as well as the possible presence of interstitial cavities when stacking and drawing cylindrical canes, may affect dramatically the fibre attenuation by introducing impurities, undesired interfaces and inducing a reshaping or deformation of the starting holes. Other problems of the stack-and-draw method may be represented by the low purity of the tubes and/or rods materials and by the difficulties in producing tubes and/or rods of the required shapes (in particular, in the case of hexagonal bodies) and dimensions and in obtaining the required pattern of holes (due for example to the difficulty in realizing geometries different from triangular when positioning rods and tubes in close-packed arrangement). Moreover, the relatively low productivity and high cost make this method not much suitable for industrial production.
A further drawback of the stack-and-draw method, described for example in US 2001/0029756, is that the outer air holes of the fibre are typically closed or are much smaller than the inner air holes. Hence, during the drawing of an optical fibre from the preform, relatively large inner air holes are transformed to an oval shape since the outer glass tubes are melted faster than the inner glass tubes due to the difference in the heat conductivity between the inner portion and the outer portion of the optical fibre preform. This type of distortion in the air holes makes the continuous mass production of holey optical fibres very difficult.
To solve the above problem, US 2001/0029756 proposes, instead of arranging the plurality of glass tubes as in the conventional stack-and-draw method, to vertically arrange the plurality of glass tubes in a gel to prevent the distortion of air holes during the drawing step of the optical fibre. In more detail, US 2001/0029756 proposes the following method for fabricating a holey optical fibre. A sol is first formed by mixing a starting material, deionized water and an additive. The sol is filled into a circular frame and gelled, and a preform rod is inserted into the center of the resulting gel. Meanwhile, a plurality of glass tubes is vertically arranged around the preform rod in the gel. Then, the gel is removed from the circular frame and dried. The dry gel is glassified through a heat application during the sintering process. Thereafter, the holey optical fibre is drawn from the holey optical fibre preform resulting from the sintering process by supplying gas into the ends of the air holes in the holey optical fibre preform while heating the other ends of the air holes, thereby preventing distorsion of air holes.
The Applicant observes that the method for fabricating a holey optical fibre proposed by US 2001/0029756 has the drawback that the holes and the core dimensions in the final fibre are limited by the inner and outer diameter of the tubes and rod employed in the assembly, which is also one of the limits of the conventional stack-and-draw method previously described.
Accordingly, the Applicant has tackled the problem of providing a process for manufacturing a micro-structured optical fibre that overcomes the above-mentioned problems of the known techniques.